1. Field of the Invention
The invention relates to a piezoelectric transformer which transduces the amplitude of an AC voltage by means of a piezoelectric effect of a piezoelectric material such as piezoelectric ceramics.
2. Related art of the Invention
A piezoelectric transformer was developed in the late 1950s. At first, such a piezoelectric transformer was developed as a step-up transformer for a high-voltage power source. However, a piezoelectric transformer has constraints on a material, such as the breaking strength of a piezoelectric ceramic material. Therefore, the development has been aborted while it has not been commercially produced on a large scale. Recently, as development of high-strength piezoelectric ceramics is advancing and the request to miniaturize and thin a portable information device such as a notebook personal computer, an electronic organizer, and a game machine are increasing, a piezoelectric transformer again attracts much attention as a step-up transformer for an inverter power source of a back light of a liquid crystal display device which is mounted such a portable information device.
For example, an inverter for a back light of a liquid crystal display device is used in a light power source of a cold cathode fluorescent lamp which serves as a back light source. The inverter must convert a low DC voltage such as 3 V, 5 V, 9 V, or 12 V due to a battery or the like, to a high voltage of a high frequency which is about 1,000 Vrms in an ignition state and about 500 Vrms in a steady state. Today, an electromagnetic winding transformer is used in an inverter for a back light. In such an electromagnetic transformer, a lateral structure having a core of a special type is used so as to satisfy the request of thinning. In order to ensure the dielectric strength, however, miniaturization and thinning are limited. Moreover, the core loss, and the winding loss due to the use of a thin copper wire are large, and hence such a transformer has a defect that the efficiency is low.
By contrast, a piezoelectric transformer is configured in the following manner. Primary (input) and secondary (output) electrodes are formed on a piezoelectric ceramic material such as lead zirconate titanate (PZT) or a piezoelectric crystal material such as lithium niobate. An AC voltage of a frequency in the vicinity of the resonance frequency of the piezoelectric transformer is applied between the primary electrodes to cause the piezoelectric transformer to mechanically vibrate. This mechanical vibration is transduced by means of the piezoelectric effect and then output from the secondary electrode as a high-voltage power. A piezoelectric transformer has features that it can be miniaturized, particularly thinned more easily than an electromagnetic transformer, and that a higher efficiency is attained.
Hereinafter, a piezoelectric transformer of the prior art will be described with reference to the drawings.
FIG. 10 is a schematic view of a Rosen-type piezoelectric transformer 101 of the prior art. This transformer has a configuration in which, on a rectangular plate 105 which is made of a piezoelectric ceramic material such as lead zirconate titanate (PZT), primary (input) electrodes 102 are formed so as to occupy a substantially half of the principal face and to be opposed to each other, and a secondary (output) electrode 103 is formed on an end face. As shown in the figure by arrows and P, the rectangular plate 105 is previously polarized in the thickness direction by using the primary electrodes 102, and in the longitudinal direction by using the secondary electrode 103. When an AC voltage of a frequency in the vicinity of the resonance frequency of the piezoelectric transformer 101 is applied between the primary electrodes 102, the piezoelectric transformer 101 produces a mechanical vibration in which expansion and contraction occur in the longitudinal direction. The mechanical vibration is transduced by means of the piezoelectric effect into a high voltage, and the high voltage is output from the secondary electrode 102.
FIG. 11 is a diagram showing a side view of the piezoelectric transformer 101 shown in FIG. 10, the displacement distribution, and the charge distribution. Referring to the figure, when a high voltage is applied to the primary electrode 102a and the secondary electrode 103 with using the primary electrode 102b as a common electrode, the primary electrode pair 102a and 102b causes the rectangular plate 105 to be polarized in the thickness direction, and the secondary electrode pair 103 and 102b causes the rectangular plate 105 to be polarized in the longitudinal direction, as shown by arrows in the figure. When, after the polarization, an AC voltage of a frequency in the vicinity of the resonance frequency of the rectangular plate 105 is applied to the electrode 102a with using the primary electrode 102b as a common electrode, the rectangular plate 105 produces a mechanical vibration in the longitudinal direction and having the displacement distribution shown in FIG. 11. This mechanical vibration is transduced by means of the piezoelectric effect into a high voltage, and the high voltage can be output from the secondary electrode pair 103 and 102b. A piezoelectric transformer of this kind was invented by Rosen, and hence is called a Rosen-type piezoelectric transformer. Among piezoelectric transformers of this type, a transformer which excites a stretching vibration of a half wave length as in the case of the longitudinal displacement distribution shown in FIG. 11 is often called a .lambda./2 mode (.lambda. indicates one wavelength) Rosen-type piezoelectric transformer. With respect to a certain driving frequency of a required high AC voltage, a .lambda./2 mode Rosen-type piezoelectric transformer is the smallest one among piezoelectric transformers in which the resonance frequency coincides with the driving frequency. Therefore, a .lambda./2 mode Rosen-type piezoelectric transformer is usually used.
In the above-described piezoelectric transformer of the prior art, on the rectangular plate 105 which is made of a piezoelectric ceramic material such as lead zirconate titanate (PZT), the primary (input) electrodes 102a and 102b are formed on a substantially half of the principal face so as to be opposed to each other, and the secondary (output) electrode 103 is formed on the end face. FIG. 12 shows frequency characteristics of the input admittance as seen from the primary side of the piezoelectric transformer shown in FIG. 10. As seen from FIG. 12, in a .lambda./2 mode Rosen-type piezoelectric transformer of the prior art, both the .lambda./2 (1/2-wavelength) vibration mode which is the fundamental vibration mode, and the .lambda. (1-wavelength) vibration mode can be strongly excited.
When the transformer is driven by an AC voltage such as a rectangular wave, containing many frequency components, therefore, also the .lambda. (1-wavelength) vibration mode is strongly excited together with the .lambda./2 (1/2-wavelength) vibration mode which is the fundamental vibration mode. As a result, distortions and stresses due to the .lambda. vibration mode are superimposed on those due to the .lambda./2 vibration mode, so that distortions and stresses of the piezoelectric transformer are increased. This produces a problem in that the characteristics are impaired and the transformer is broken, thereby lowering the reliability.
Japanese patent publication (Unexamined) No. HEI9-181371 discloses a piezoelectric transformer device in which a primary electrode elongates from one end portion in the longitudinal direction so as to exceed the center portion, and a secondary electrode is formed on the other end portion. FIG. 13 shows the piezoelectric transformer device, and FIG. 14 shows frequency characteristics of the input admittance as seen from the primary side of the piezoelectric transformer shown in FIG. 13. In this way, the formation of the primary electrode which exceeds the center portion causes induced charges of the .lambda. vibration mode to be partly canceled. In this structure, however, the suppression effect of the .lambda. vibration mode is small. Actually, also a vibration of the .lambda. (1-wavelength) vibration mode is therefore excited. Consequently, it is impossible to strongly excite only the .lambda./2 (1/2-wavelength) vibration mode which is the fundamental vibration mode.
As a result, in the same manner as a Rosen-type piezoelectric transformer of the prior art, the output AC voltage is not a sinusoidal wave but an AC voltage containing high frequency components. Depending on the kind of the load, the sinusoidal-wave driving may be required. The existence of such high frequency components produces problems in that the reliability of the load and the circuit efficiency are lowered, and that undesired electromagnetic waves are radiated to the outside.
Japanese patent publication (Unexamined) No. HEI9-74236 discloses a center-driven piezoelectric transformer device which is a .lambda./2 mode piezoelectric transformer. FIG. 15 shows the piezoelectric transformer device having a structure in which a driving portion that is polarized in the thickness direction is disposed in the center portion and a generating portion that is polarized toward the end and in the longitudinal direction is disposed in each of the sides of the driving portion. FIG. 16 shows frequency characteristics of the input admittance as seen from the primary side of the piezoelectric transformer shown in FIG. 15. As seen from FIG. 16, a .lambda./2 mode Rosen-type piezoelectric transformer of this type is excellent in that only the .lambda./2 (1/2-wavelength) vibration mode which is the fundamental vibration mode can be strongly excited.
In a piezoelectric transformer of this structure, however, secondary output terminals are located at the ends of the device, respectively. When both the terminals are to be connected to each other, therefore, a wiring for a high voltage must be formed on the circuit. This produces a problem in that undesired electromagnetic waves are radiated. Furthermore, the characteristics are fluctuated by a stray capacity due to this wiring. The electric connection must be made at portions respectively corresponding to two antinodes of the vibration, thereby producing a problem in that a vibration is hindered and hence the efficiency is lowered.
In order to avoid this problem, only one of the output terminals may be used. In this case, for the sake of the reliability and safety of the device, the other secondary terminal must be connected to a load or to the ground. This produces problems in that the circuit efficiency is lowered by the connection of a load, and that the different output voltages at the ends lower the reliability of the piezoelectric transformer device.